The present invention relates to an internal pump for a nuclear reactor and, more particularly, to a wet motor type reactor internal pump which is disposed in the bottom of a reactor pressure vessel for circulating the reactor coolant. The present invention is especially applied to a rotary portion floating device for the reactor internal pump which has its rotor weight increased by using a flywheel at its rotary portion.
In a known reactor internal pump of the prior art, as disclosed in Japanese Published Patent Application No. 211595/1983, there is fixed on the shaft in a motor chamber a thrust disc acting as an auxiliary impeller. A fixed orifice is formed between the outer circumference of the thrust disc and an external ring facing the same whereas a variable orifice is formed between the inner lower face of the thrust disc and an internal ring facing the same, to define a pressure regulating chamber between the two orifices.
In the prior art technology described above, the upward thrust is balanced by finely adjusting the pressure in the pressure regulating chamber through the variable orifice. Since the reactor internal pump of the prior art has no flywheel, it is not concerned with the substantial weight increase due to the flywheels.
Reactor internal pumps may be required to have a high inertia by the system used in a plant. For this high inertia, a flywheel is added to the motor unit so that the weight of the rotary portion is accordingly increased. This increase of weight leads to an increase in the load upon the lower thrust bearing, thus raising a problem that the bearing lifetime is adversely affected.